The demand for locating the position of a mobile wireless device, such as a cellular phone, is on the rise. For instance, a mandate by the Federal Communications Commission (FCC) requires that the location of a caller dialing 911 on his/her cellular phone be identified with an accuracy of 400 feet when the 911 call is received. A widely known method for determining the position of a mobile wireless device uses the information obtained from the Global Positioning System (GPS).
The GPS is a satellite-based navigational system formed by a network of satellites broadcasting pseudo-random noise (PRN) codes modulated on a carrier band. The GPS satellites transmit signals from which mobile GPS receivers may estimate their locations. Each GPS satellite transmits signals using two carrier signals. The first carrier signal is modulated using two PRN codes, namely a coarse acquisition (C/A) code, and a precise (P) code. Each GPS satellite uses different PRN codes to distinguish it from the other satellites of the GPS.
To determine the location of a GPS receiver, acquisition and tracking of at least four satellite signals are required. GPS signal acquisition often involves computing the correlation between the received GPS signals and the C/A code of associated satellites at various phase offsets and Doppler-shifted frequencies. Following signal acquisition, a signal tracking process decodes the signals from the identified satellites at the phase offsets and Doppler-shifted frequencies. During the signal tracking phase, navigation data is received from the identified satellites. Embedded in the navigation data transmitted by the GPS satellites are data related to satellite positioning as well as clock timing (i.e., time stamp), commonly referred to as ephemeris data, from which the position of the GPS receiver is detected. Many techniques have been developed to acquire and track GPS signals and to read the ephemeris data so as to detect the location of the GPS receiver.
GPS-based position detection systems, however, have a number of disadvantages. One such disadvantage is that a GPS receiver must have a clear and unobstructed view of at least four GPS satellites in order to have its position detected accurately. Therefore, if a user of a GPS receiver is, for example, in a wooded or an urban area containing tall structures, the user may not have an obstructed view of the required number of satellites to be able to detect its position. The problem may further be compounded if the user is indoors.
To detect the position of a wireless receiver when fewer than four satellites are in clear view, algorithms have been developed that use the CDMA signals transmitted by the ground base stations. Communication between a wireless device and a base station is often established by a forward link via which signals are transmitted from the base station to the wireless device, and a reverse link via which the signals transmitted from the wireless device are received by the base station.
One known algorithm developed for detecting the position of a mobile wireless device using ground base stations is referred to as Advanced Forward Link Trilateration (AFLT). To detect the position of a mobile wireless device using AFLT, the location of the base stations disposed in the wireless network, the transmission times of the signals from the base stations as well as the arrival times of the signals at the mobile wireless device are required.
A mobile wireless device is often not in the line-of-sight of a base station with which it is in communication. Accordingly, the transmitted CDMA signals are often deflected by many objects and thus experience additional delays before they are received by the mobile wireless device. Conventional AFLT algorithms, however, assume that a mobile wireless device in communication with a base station is in the line-of-sight of that base station and thus detect a position that may include an unacceptable degree of inaccuracy.
Accordingly, there is a need for a technique that detects the position of a mobile wireless device using the CDMA signals transmitted by the base stations and which accounts for any additional delays that may have been caused by the deflections of these signals.